Light beam scanners used for input or output of a high resolution image require a dimensionally-stable beam source and beam collimating assembly, sometimes termed an "optical head". Such an assembly typically includes a light beam source, a collimating lens, and a means for maintaining a constant beam focal distance over the operating temperature range. In order to achieve adequate optical performance, the optical head must maintain a predetermined beam quality over a wide ambient temperature range.
In conventional apparatus, the beam source and collimating lens are mounted in a mechanical structure that attempts to maintain the beam focal length while the apparatus undergoes temperature-induced structural changes. Such athermalization (i.e., thermal compensation) is effected either passively or actively. Passive systems rely on the differences in the coefficients of thermal expansion of various movable elements in the optical system such that there is no net focal shift over temperature. Active systems compensate in the same manner, but employ actuators to effect movement of the optical elements. However, the foregoing approaches have been found to be more costly and complex, and offer less precision and less range of adjustment, than is desired for certain beam scanning applications.
Additionally, some conventional optical head assemblies are supported by an orthogonal four-point mechanical system, whereby the collimating optical components are located on a solid yoke that is separated from the beam source by four motor devices and suspended by four respective pin hinges. All motion of the yoke is constrained by the action of four respective hinges. Any movement of the yoke requires a push-pull operation of at least two opposing motors and hinges in tandem (e.g., one upward, and one downward) to achieve a particular, albeit limited, amount of repositioning of the collimating optical components. An optical head assembly that offers more freedom of motion of the collimating optics, and a simpler control apparatus for effecting such motion, would be advantageous.